1. Field of the Invention
The present invention relates to medical diagnostic and treatment methods and apparatus.
2. Discussion of the Related Art
An electroencephalogram (EEG) is a record of specific brain wave patterns in a patient. EEG systems permit the recording of the brain wave patterns. An EEG system typically includes a plurality of conductive electrodes that are placed on a patient's scalp. These electrodes are typically metal and are connected to a preamplifier that processes the signals detected by the electrodes and provides amplified signals to an EEG machine. The EEG machine contains hardware and software that interprets the signals to provide a visual display of the brain wave activity detected by the electrodes. This brain wave activity is typically displayed on a strip chart recorder or computer monitor.
Transcranial magnetic stimulation (TMS) is a technique for stimulating the human brain non-invasively. TMS uses the principle of inductance to get electrical energy across the scalp and skull without the pain of direct percutaneous electrical stimulation. It involves placing a coil of wire on the scalp and passing a powerful and rapidly changing current through it. This produces a magnetic field which passes unimpeded and relatively painlessly through the tissues of the head. The peak strength of the magnetic field is related to the magnitude of the current and the number of turns of wire in the coil. This magnetic field, in turn, induces a much weaker electrical current in the brain. The strength of the induced current is a function of the rate of change of the magnetic field, which is determined by the rate of change of the current in the coil. In order to induce enough current to depolarize neurons in the brain, the current passed through the stimulating coil must start and stop or reverse its direction within a few hundred microseconds.
TMS is currently used in several different forms. In a first form, called single-pulse TMS, a single pulse of magnetic energy is delivered from the coil to the patient. Repetitive TMS or rTMS, refers to the delivery of a train of pulses delivered over a particular time period. An example of rTMS could be a train of pulses having a 10 Hz repetition rate that lasts for approximately 8 to 10 seconds. In a typical application, this train of pulses is repeated every 30 seconds for up to 20 or 30 minutes.
In order to monitor the safety and efficacy of a TMS application, it would be desirable to monitor a patient's EEG during a TMS session. However, monitoring a patient's EEG during a TMS pulse presents problems because at least the preamplifiers used in current EEG systems experience saturation caused by the magnetic field generated by the TMS system. Since the electrodes used to monitor the EEG are typically in close proximity to the TMS coil, the magnetic pulse induces a signal in one or more of the EEG electrodes which causes the EEG preamplifiers to saturate. Typical preamplifiers used in EEG systems take a relatively long time to recover after being saturated by a TMS pulse.
One currently available EEG system has preamplifiers that recover, i.e., return to normal operation, after 150 milliseconds after the TMS pulse has ended. This presents a problem since, in a typical TMS pulse train, the time interval between pulses is approximately 100 milliseconds. If the amplifiers come out of saturation after 150 milliseconds, the next TMS pulse has already resaturated the amplifier and therefore it is simply not possible to record or observe the EEG during a typical TMS pulse train.
Another system for monitoring EEG during TMS includes amplifiers in the EEG system that use a sample-and-hold circuit to pin the amplifier to a constant level during the TMS pulse. The amplifiers are said to recover within 100 microseconds after the end of the TMS pulse. Although this system appears to allow monitoring of the EEG within a short time after the end of a TMS pulse, additional gating and synchronizing circuitry is necessary to control the operation of the EEG amplifiers with respect to the TMS system. Additional gating and sampling circuitry is undesirable because it requires additional circuitry and because it can be complicated.
An additional complication that occurs when a patient's EEG is monitored during TMS occurs because of the use of metal electrodes to sense EEG signals. Large eddy currents induced by the TMS pulse or pulses in the metal electrodes can cause localized heating that may result in burns to a patient's scalp. This presents a safety hazard.
Therefore, it would be desirable to provide a system that allows a patient's EEG to be monitored during a TMS pulse that overcomes at least these problems.